The invention relates generally to a magnetostatic wave filter. More particularly, the invention relates to a magnetostatic wave filter capable of controlling the property of flatness and the width of the band-pass that are characteristics within a transfer band of a filter, by adjusting the pattern shape of a transformation section in input and output section.
In a conventional magnetostatic wave filter, the distance between a line and its neighboring line is constant in the longitudinal direction of the line even when a line having the constant size or multiple lines are used with various shapes of input/output electrodes. Also, a A structure having a magnetic thin film at one side is used. Therefore, there are problems that the band-pass width of the filter is narrow and a band-pass ripple occurs severely.
In addition, in order to expand the band-pass width and reduce the ripple, the thickness of a ferromagnetic thin film must be increased, thus resulting high manufacturing cost.
Also, as there is not used a metal shield film for electrically isolating an input and an output in a flat structure, the coupling of the input and the output occurs outside the pass frequency band. Thus, there is a problem that the attenuation rate of the magnetostatic wave filter is poor.
Prior references relating to conventional magnetostatic wave filters for overcoming these problems are now explained.
First, there is U.S. Pat. No. 5,663,698, issued to xe2x80x98Japan Murataxe2x80x99, invented by xe2x80x98Takekazu Okada, Satoru Kanaya and Shinichiro Ichiguchixe2x80x99 and entitled xe2x80x98Magnetostatic Wave Device having Slanted End Portionsxe2x80x99.
In order to implement a miniature and low-cost magnetostatic wave filter, the prior patent implements the filter by cutting the ends of a YIG (yttrium, iron, garnet) thin film (the YIG thin film grown on a GGG substrate, see xe2x80x9c120xe2x80x9d shown in FIG. 1) formed on transducers at a constant angle or by putting absorbers on the surface of which is rugged and having a large loss at an end portion of the YIG thin film in order to terminate magnetostatic waves. Therefore, it can reduce the usage of magnetic thin films and accomplish miniaturization, thus resulting a low cost magnetostatic wave filter.
Examining particularly the structure of this prior patent, transducers are formed on a dielectric substrate, having the electrode patterns having the same widths and are constituted by a single line. The end portion of the YIG thin film is slanted (see xe2x80x9c128axe2x80x9d and xe2x80x9c128bxe2x80x9d shown in FIG. 1) or has absorbers on it. Also, the YIG thin film is located at one side of the GGG (gadolinium, gallium, garnet) substrate, and the end portion of the YIG thin film is slanted or has absorbers on it, thus reducing the amount reflected from the end portion of the YIG thin film.
There is another U.S. Pat. No. 4,983,937, issued to xe2x80x98Japan Hitachxe2x80x99, invented by xe2x80x98Yasnaki Kinoshita, Sadami Kubota and Shigern Takedaxe2x80x99 and entitled xe2x80x98Magnetostatic Wave Band-Pass Filerxe2x80x99.
The prior patent relates to the field of a magnetostatic wave filter in which a signal processing filter for processing a high frequency signal such as a high frequency device is implemented using magnetostatic waves. The magnetostatic wave filter has a magnetostatic filter formed by using a plurality of resonators and a photolithography technology, so that a good frequency response characteristic and a good reappearance can be provided and it is suited for mass production.
In order to accomplish this object, after a YIG film is grown on a GGG substrate, an input/output electrode pattern is formed on the surface of the YIG film or the rear surface of the GGG substrate, using an etching technology. If a high frequency signal is incident to the magnetized YIG film, magnetostatic waves are excited. The excited magnetostatic waves are reflected from the end portion of the YIG film and the reflected waves produce standing waves between both ends of the YIG film, so that the YIG film is resonated by the standing waves. Therefore, if the output electrode is formed in the film parallel to the end portion of the YIG film, high frequency components excited by means of the magnetostatic waves are outputted. At this time, the input/output electrodes may be formed not of a single line but a plurality of electrodes.
In other words, the prior patent has transducers formed on the YIG film or the GGG substrate, transducer electrode patterns having the same widths, the transducer longitudinal direction parallel to the YIG longitudinal direction, and the input/output electrodes made of a plurality of lines, not a single line.
If a filter is manufactured using the technology, proposed in the prior patent, it could obtain a good frequency response characteristic and a good reappearance and is also suited for a mass production.
Further, there is U.S. Pat. No. 4,199,737, issued to xe2x80x98US Westing housexe2x80x99, invented by xe2x80x98Ralph W. Patterson, Terence W. O""keeffe and John D. Adamxe2x80x99 and entitled xe2x80x98Magnetostatic Wave Devicexe2x80x99.
In order to provide a magnetostatic wave filter capable of processing a signal in a high frequency region (in the region of about 20 GHz) and having a low loss and that can be manufactured by a photolithography method, structure is proposed for designing transducers having various shapes and for exciting magnetostatic waves. Thus, it can reduce reflection of a magnetostatic wave at a desired frequency and can design a filter having a relatively narrow bandwidth.
In other words, the transducer has transducer electrode patterns with the same width. The transducer is weighed so that the length of the transducer can be varied and two transducers are paired to reduce reflection in xc2xd wavelength. Also, the YIG thin film is located at one side of the GGG substrate.
Also, there is an article published in xe2x80x98Ultrasonics Symposiumxe2x80x99 (written by xe2x80x98Takuro Koike and Hiroaki Nakazawaxe2x80x99, 1994) entitled xe2x80x98A new method for controlling Resonant frequencies of Straightedge MSW resonatorsxe2x80x99.
In the article, when the bandwidth required in a microwave frequency is narrow, it proposes a method of controlling the frequency in a line-shape magnetostatic wave resonator. In order to accomplish this object, the transducer has transducer electrode patterns with the same widths. It further includes a PIN diode serially connected to an end of the transducer and has a line-shape transducer. Also, the YIG thin film is located at one side of the GGG substrate.
In addition, there is an article published in xe2x80x98IEICE Trans ELECTRON.xe2x80x99(Vol. E77-C, No.2) (xe2x80x98Yatsuya Omori, K. Yashiro and Sumio Ohkawaxe2x80x99) entitled xe2x80x98A study on magnetostatic surface wave excitation by Microstripxe2x80x99.
The above article proposes a method by which the current density and the radiating resistance in the transducer are calculated, thus implementing a magnetostatic wave filter capable of processing signals in a microwave frequency.
In order to accomplish these objects, it calculates the radiating resistance of the transducer by predicting the current distribution by interpreting the distribution of the electric field in the transducer in a single line using a TE mode and by then predicting the flow of current. When a filter is constituted using a magnetostatic wave, the results are used to design the transducer so that the impedance match can be made well.
In the above mentioned conventional magnetostatic wave filters, an input/output is formed on a magnetically active ferromagnetic thin film that is formed on a magnetically inactive dielectric substrate, or after forming an input/output electrode on a magnetically inactive dielectric substrate, a ferromagnetic thin film is located on it. Therefore, an external magnetic field is applied to the central frequency to implement the function of the filter through transformation and propagation of energy. Also, the shape of the conventional input/output electrode has the shape connected by a line in which the size of the exciting line is uniform. In addition, even when multiple lines are used, the distance between the line and its neighboring line is constant in the longitudinal direction of the line. As such, because the prior devices do not consider variations in the wavelengths per frequencies in the pass frequency bandwidth of a desired filter, there is a problem that a ripple is severely occurs in the characteristic within the pass-band of the filter.
In addition, the prior devices use a structure in which a ferromagnetic thin film is formed at one side of a magnetically inactive substrate and do not use a metal shield film for electrically separating an input section and an output section in a flat structure. Therefore, there is a problem that transfer of energy occurs since coupling of the input section and the output section occurs outside of the pass frequency band.
An exemplary conventional magnetostatic wave filter will be now explained with reference to FIGS. 1 and 2.
FIG. 1 is a plane view showing a structure of a conventional magnetostatic wave filter and FIG. 2 is a front view showing a structure of the magnetostatic wave filter shown in FIG. 1.
The conventional magnetostatic wave filter includes an input transfer line 116a and an output transfer line 116b, both of which have the constant line widths, formed on the other side of a dielectric substrate 180, one side 190 of which is grounded, an input energy transformer 117a comprised of a plurality of straight lines having the line widths for producing the energy transformation of an electromagnetic wave and a magnetostatic wave, the distance between neighboring lines is uniform, and a magnetically active ferromagnet 124 formed on a magnetically inactive substrate 123.
An external DC magnetic field having the intensity higher than saturation susceptibility of a ferromagnetic substance is applied to the conventional magnetostatic wave filter in an adequate direction, so that a magnetically active ferromagnet can be completely saturated. Thus, if an electromagnetic wave of a frequency band by which the saturated ferromagnetic substance can absorb is transferred to the energy transformer 117a, an electromagnetic wave is magnetically couplcd in the energy transformer 117a to produce a magnetostatic wave. At this time, after the produced magnetostatic wave is propagated at a certain length by the medium of the magnetized ferromagnetic substance, they are inversely transformed in the output transformer so that transfer of energy is caused.
Referring now to FIG. 3, a line structure of an input energy transformer 117a and an output energy transformer 117b, used in the conventional magnetostatic wave filter will be now explained.
The energy transformation line of the electromagnetic wave and the magnetostatic wave in FIG. 3 is implemented using a single line having a uniform width in the direction along which current flows. The drawing shows a transformer in which a number of lines are consecutively arranged by given distances g1 and g2. The reason for using these transforming lines is for selecting a specific one frequency and is advantageous in obtaining a narrow band-pass transfer characteristic. However, there is a problem that the transferred band-pass is crouched because efficiency to select all of the given frequency band-pass is degraded.
Also, as the shield film for preventing an electromagnetic coupling between the input section and the output section in FIG. 1 does not exist in the middle of the propagation passage, the transfer of energy due to the coupling of an electromagnetic wave in the region in which the magnetostatic wave is generated and other regions in which the magnetostatic wave is not generated and the propagation of energy due to transfer of the magnetostatic wave occurs together. Thus, the value outside the transfer band is significantly increased, to thereby degrade the frequency selectivity of the magnetostatic wave filter.
The disclosed embodiments of the present invention increase the transfer frequency bandwidth of a magnetostatic wave filter and reduce a ripple effect by applying a line shape in which the width of an exciting line varies in a longitudinal direction and a line shape in which the distance between neighboring lines varies in the longitudinal direction when multiple lines are used to an input electrode and an output electrode, respectively.
Further, the disclosed embodiments of the present invention reduce the coupling between an input section and an output section along the propagation length of the shield film by inserting a ground shield film through which only multiple ferromagnetic thin film can pass the magnetostatic waves between the input section and the output section and to also reduce loss of energy within the band-pass by reflecting the energy toward an opposite direction passing through a transducer using a magnetostatic wave reflector.
In order to accomplish the foregoing, a magnetostatic wave filter having a ferromagnetic substance stacked on one side of a dielectric substrate, one side of which is grounded, is provided. The filter includes an input section having an input transfer line and an input energy transformer for causing the energy transformation of an electromagnetic wave and a A magnetostatic wave, and an output section having an output energy transformer for causing the energy transformation of the electromagnetic wave and the magneto static wave and an output transfer line, on the ferromagnetic substance. The width of exciting lines in the input energy transformer and the output energy transformer varies in dimension along a longitudinal direction. The filter further includes a shield film formed of a grounded conductive film having holes through which a magnetic thin film can pass between the input section and the output section, for preventing coupling of an electromagnetic wave of the input section and the output section.
Also, according to another embodiment of the present invention, there is provided a magnetostatic wave filter including an input section and an output section. The input section includes an input transfer line for performing a transfer line function of an electromagnetic wave energy, and an input energy transformer (transducer) consisting of a single line or multiple lines, the width of the line changing in a horizontal direction in order to transform the energy between an electromagnetic wave and a magnetostatic wave using a signal received from the input transfer line. Also, the output section includes an output energy transformer consisting of a single line or multiple lines, the width of the line changing in a horizontal direction in order to transform the energy between an electromagnetic wave and a magnetostatic wave, and an output transfer line for receiving the transformed electromagnetic wave from the output energy transformer to perform a transfer line function. The filter further includes a shield film formed of a grounded conductive film having holes through which a magnetic thin film can pass between the input section and the output section, for preventing coupling of an electromagnetic wave of the input section and the output section.